Overall, 26 percent of U.S. men and 28 percent of U.S. women already are obese by about age 36, according to a new University of North Carolina at Chapel Hill study of adult weight gain among different ethnic groups, races and sexes.

For still-unknown reasons, black women become obese more than twice as fast as white women, and the rate for Hispanic women is about midway between the two. U.S. men of different races and ethnic groups also put on pounds at varying rates.

"We found Hispanic men became obese 2.5 times faster than U.S. men of European ancestry," said Dr. Kathleen M. McTigue, a Robert Wood Johnson clinical scholar at the UNC School of Medicine. "We saw no difference in the rate of obesity development between black and non-Hispanic white men until after age 28 when black men in this country became obese 2.2 times more rapidly than white men."

A report on the new study appears in the June 18 issue of Annals of Internal Medicine, a professional journal. Besides McTigue, authors are Drs. Joanne M. Garrett, associate professor of medicine; and Barry M. Popkin, professor of nutrition of the UNC School of Public Health.

Researchers analyzed information over time on 9,179 people born between 1957 and 1964 and enrolled in the National Longitudinal Survey of Youth beginning in 1979.

More than 80 percent of those who were obese by about age 36 did not become obese until after ages 20 to 22, although many began gaining excess weight earlier, McTigue said.

"Based on their gender, ethnicity and body mass index at ages 20 to 22, we could fairly accurately predict who would be obese at ages 35 to 37," she said.

Overall, the prevalence of obesity in U.S. adults between ages 20 and 74 doubled during the past 40 years, from 13 percent to 27 percent of the population, McTigue said. Sixty-one percent of U.S. adults now are either obese or overweight.

"Obesity is important for health, and, as health-care professionals, we need to pay more attention to it," she said. "In the group we studied, there was substantial obesity at ages much younger than most of obesity's health complications tend to occur. Early intervention with such people has the potential to prevent significant illness and should not be overlooked."

Equally important, the physician said, is preventing obesity in the first place and focusing more on children and people just entering adulthood who are only slightly or moderately overweight.

"Since African-American and Hispanic young adults are at particular risk for obesity, we also need to better understand ethnic differences in weight development so that we can design effective interventions," she said.

Obesity receives increasing attention nowadays because it has become so prevalent in U.S. society, McTigue said. The condition is an important risk factor for four of the six leading causes of death in this country -- heart disease, certain cancers, stroke and diabetes. It also contributes to less deadly but still troublesome osteoarthritis, obstructive sleep apnea and diminished mobility.